June 18, 2011, 8:38 p.m.
posted by lordnikon
Another way to convert an image to grayscale is to select one of the channels that make up the image. Each color image has three or four color channels (depending upon if the image is set in RGB or CMYK mode). Each one of these channels contains specific color information. For instance, an image set in RGB mode has red, green, and blue channels. Channels are themselves grayscale images. (If you’re interested in finding out more about channels, take a look at Technique 52.)
Take a look at the Channels palette shown in Figure. The Arches National Park photo that I’m using for these examples is set in RGB color mode, so you see three individual color channels. (At the top of the Channels palette is the composite channel that shows the three channels combined.)
To select an individual channel for use as a grayscale image, click each channel in the Channels palette to view it in the image window. When you decide on the channel you want to use, drag the other channels to the trashcan icon at the bottom of the Channels palette.
Figure shows the three channels that make up the Arches National Park photograph. I like the green channel best, so I’ll discard the other channels. Notice how fine the channel detail is in the green channel in Figure when compared to the grayscale conversion in Figure. Also, the overall cast of the green grayscale channel is brighter than the grayscale conversion.